The oil sands located in northern Alberta, Canada contain heavy bitumen with a gravity of approximately 8 API, in concentrations of 6 to 14 wt. %. The Alberta oil sands form one of the world's largest known sources of oil.
A considerable amount of this resource is accessible by surface mining methods, and major mining of these oil sands takes place. However, the costs of extracting, treating and up-grading bitumen are high. Accordingly it is desirable to improve the process steps to maximize bitumen recovery from the oil sands which are mined.
Techniques for the surface extraction of bitumen from oil sands are well known in the industry. The oil sands are mined and crushed to form a crushed ore (called “ore” or “oil sand ore” in this application).
An established commercial method for the processing of oil sand ore is the hot or warm water extraction process. This is sometimes known as the “Clark Process”, although many variants of it now exist. This extraction process treats the ore with steam or water or a mixture of the two with agitation in air to produce an aerated bitumen froth. The temperature of treatment varies, but is usually in the range of 40° C. to about 85° C. or higher. This process is sometimes called a “warm water” process when the water temperature is below about 50° C., and a “hot water” process when the temperature is higher, but both the warm and hot water processes will be described collectively in this application as a “hot water process”. A typical composition of the bitumen froth from such a process (excluding the air which forms it into a froth) is approximately 60 to 65 wt. % bitumen, 30 to 40 wt. % water and 5 to 10 wt. % minerals. The minerals are present as small solid particles.
It is known to add sodium hydroxide (NaOH) along with the hot water or steam in the hot water bitumen extraction process. This is done when there is a high “fines content” (which is defined in the industry as the fraction of solid particles of less than 44 micrometers in size), or when there is a low bitumen content. Generally, if there is a fines content above about 10-15% by weight, some sodium hydroxide is added, and the greater the fines content, the greater the amount of sodium hydroxide which is added. Also, sodium hydroxide can be added to increase bitumen extraction when the ore contains only a small percentage of bitumen. Low bitumen content and high fines content are often found in the same ore. “Very high grade” ore (which is defined as ore containing more than 12% by weight of bitumen) or “high grade” ore (which is defined as ore containing 11-12% bitumen, do not have sodium hydroxide added to them during hot water bitumen extraction. The amount of sodium hydroxide present in commercial hot water bitumen extraction processes can vary from no sodium hydroxide at all (for rich ores containing a lot of bitumen and relatively small amounts of fines) to approximately 0.03 wt. % of sodium hydroxide based on the weight of the ore for very low-grade ores which contain little bitumen and large amounts of fines.
It is recognized in the industry that sodium hydroxide should be used as little as possible in the hot water treatment process, having regard to the need for controlling fines and extracting bitumen from poor ores. This is because sodium hydroxide addition increases the cost of the treatment process. Also, it is known that sodium hydroxide delays seriously the settling rate of tailings (the mixture of minerals, clay and water which is left over after extraction of the bitumen). This increases the difficulty of managing the disposal of the tailings. Also, it is found that addition of sodium hydroxide beyond a certain optimum level for any particular ore does not increase bitumen production: in fact, it may reduce it.
Although the role of sodium hydroxide in bitumen froth production is not well understood, a few studies have linked it to the production of natural surfactants. It has been said that aged bitumen may be deficient in surfactants, and sodium hydroxide could cause some to be generated. See articles by Schramm et al., (1984) AOSTRA Journal of Research, vol 1, page 10, and (1987) AOSTRA Journal of Research, vol 3, page 215. However, this work has not led to any method of identifying bitumen which is lacking in such surfactants, or to any practical process of treating such ores. Further, the amount of sodium hydroxide which is suggested in these articles for addition to high grade ore is low, being about 0.01 weight %.
Generally, oil sand treatment by the hot water bitumen extraction process is quite effective, and leads to good recovery of bitumen as bitumen froth. It is sometimes found, however, that, as the hot water process is running as a continuous ore treatment process, the bitumen froth output from it increases in density, because the ratio of mineral and water content in the froth increases markedly and the bitumen content decreases. This can lead to plugging up of the froth treatment equipment, such as centrifuges, and hence force the process to shut down. When the problem is not severe enough for a shutdown, it can still lead to reduced recovery of bitumen. Similarly, batch processes, even those running high or very high grade ores, can sometimes give rise to high density froth without any obvious reason.
Inspection of the ores which are implicated in the increased density problems in the hot water process does not give obvious indicia which are different from other ores. Many are high grade or very high grade ores. Sometimes, but not always, the ores show signs of oxidation (for example, elevated iron, calcium or magnesium content indicative of a oxidation of iron sulphide to iron sulphate). However many of the ores which give high densities of froth or low bitumen recoveries do not show these features.
The ores implicated in the increased density problems are not “poor” ores in the classical sense of the term. Their fines content and bitumen content do not differ significantly from other ores from the same ore body which process satisfactorily in the hot water process. For example, high density froth problems can occur even with ores having 14% bitumen content or more, which is extremely high grade ore. Typically, ore with this bitumen content would give a froth with under 10% mineral. Much higher mineral concentrations than this are observed with the increased density ores, leading to plugging of equipment.